The invention relates to an off-drawing apparatus for tubular webs of plastic film made by an extruder with a fixedly disposed blow head.
In the production of tubular webs of plastic film by the blowing method variations in thickness are unavoidable. They are due, for example, to the usual production tolerances in the blow head of the extruder used in making the film, in the gauging system through which the tubular web of expanded, round cross section passes after extrusion, or due to the internal and external cooling or to other external circumstances. Unless appropriate measures are taken, when such tubular film webs are wound in the flattened state in a winding apparatus, raised rings would be produced on the roll of film due to the addition of the irregularities in thickness, and these would result in a permanent deformation of the film in those areas and would interfere with the printing and further processing of the tubular film.
As an appropriate countermeasure for preventing the formation of raised rings on the film roll, so-called reversing off-drawing apparatus have been developed for the tubular web, in which the reversing movement of the parts of the apparatus that are involved produces a periodical migration of the creases formed when the tubular web is flattened. This results in a corresponding distribution of any thickness irregularities over the width of the flattened tubular web. In the winding of the flattened tubular web that then takes place in a winding apparatus, the formation of raised rings on the roll is prevented by the addition of thickness irregularities of the film. Obviously, the angle of the reversing movement must be made sufficiently great.
In a known generic apparatus (German Fed. Pat. No. 20 35 584) the lay-flat apparatus with its pinch rollers is swiveled reversingly over approximately 360.degree. about the axis of the delivered tubular web, and it has been found that this swivel angle is proportional to a reversing movement for the majority of applications of a reversing off-drawing apparatus, while in addition to the two pulley rolls for turning the web by about 180.degree., two overturning bars are provided in tandem for turning the web about 180.degree. with simultaneous change of direction, successively and alternately with one another. On the other hand, in applications in which the swivel angle of the reversing movement of the lay-flat apparatus with its pinch rolls amounts to less than 360.degree., a single overturning bar between the two pulley rolls will suffice for the handling of the tubular web in the reversing part of the off-drawing apparatus.
In the known off-drawing apparatus constituting the genus, the supporting of the pulley rolls and overturning bars in the apparatus for swiveling is performed such that the axes of the pulley rolls and overturning bars are tangential to circles about the axis of rotation of the lay-flat apparatus, the pulley rolls being situated in every possible working position, radially outside of the parts of the overturning bars around which the tubular web passes. The swivel angles of the pulley rolls and overturning bars decrease with increasing distance from the lay-flat apparatus, and the tube of film is guided between the pulley rolls and overturning bars as well as the stationary conveyor means for carrying away the flattened tubular web in horizontal planes parallel to one another. In this manner a low structural height of the reversing system is achieved, and also, by making the radius of the circle described by the swiveling path of the axes of the overturning bars amount to the magnitude of .pi./4.times.overturning bar diameter, any lateral skewing of the tubular plastic film web during the swiveling of the reversing system is counteracted. At the same time it is found that the intersection of the center axes of the tubular web strands running to and from the overturning bar lies in the central swiveling axis of the reversing system. During the reversing swiveling movement this assures that the tubular web strands will open and close in a scissor-like manner around the swivel axis, so that, theoretically, the idea is that equal friction forces opposed to one another will act symmetrically on both tubular web strands and thus axial shifting of the tubular web on the overturning bars during the reversing operation, due to unilaterally acting friction forces, will be prevented.
In the known off-drawing apparatus the lay-flat apparatus, the pulley rolls and the overturning bars are all supported on a relatively thin central shaft for performing their reversing swiveling movements, and this shaft runs in the vertical prolongation of the axis of the tubular web and is journaled in the frame of the apparatus. The entire weight of the reversing system is thus carried by the central shaft, with the result that the reversing system is very much liable to fluctuations and oscillation, which are created very rapidly, especially in the case of an off-drawing apparatus set at a high level due to a long vertical off-drawing length, when for example an operator walks on the catwalk of the off-drawing apparatus. Furthermore, the installation and maintenance of the known reversing system, and especially also the introduction of the tubular web into the off-drawing apparatus at the beginning of an extrusion is time-consuming and difficult on account of the shaft located in the center of the apparatus in prolongation of the axis of the tubular web, since this central area is difficult to reach and is occupied in part by the tubular web being carried through the apparatus.
It is therefore the aim of the invention to create an off-drawing apparatus of the generic type, while retaining the motion of the reversing system and the same controlled guidance of the tubular web from a stationary tubular film blow head through the reversing off-drawing apparatus to a stationary winding apparatus, in which the reversing system will have an improved stability avoiding unwanted fluctuations and oscillation, while at the same time facilitating the installation and maintenance of the system as well as the introduction of the tubular web.
This aim is achieved according to the invention by providing a configuration in which reversingly swiveling parts of the apparatus are supported on swivel rings whose inside diameter is at least equal to the maximum working width of the apparatus, which corresponds to the greatest possible width of the flattened tubular web that can be handled in the apparatus, i.e., the inside diameter of the swivel rings is approximately equal to the length of the pulley rolls. On the basis of these dimensional conditions, the stack of swivel rings formed by the individual rings constitutes a stable structure in which the swivel rings are supported on one another only through the interposed rolling bearings and, in regard to the bearing force, make the swivel ring stack serve as an axial bearing. This configuration virtually excludes any undesired fluctuations or oscillation when a person is walking on a catwalk or access bridge mounted on the frame of the apparatus. On account of its large inside diameter, which is equal in all of the swivel rings, the central area of the reversing system of the off-drawing apparatus, in which the vertically rising tubular web runs into the system, is kept free of any parts of apparatus which might make installation and maintenance difficult or interfere with or complicate the introduction of the tubular web at the beginning of an extrusion.
Numerous additional features and advantages of the invention will be found in the subordinate claims and in the description that follows, in conjunction with the drawing in which an embodiment of the subject matter of the invention is represented diagrammatically.